Chris this is highly subjective. You will be limited by the slowest person in your group. 12 miles with 3,500 elevation gain shouldn't be too difficult for most people. But what is your starting elevation? Some people do poorly at 5,000' or higher.
This is a matter of everyone knowing their body, which is gained by hiking experience. Some people could do that first day in 6 hours or much less, others it would take 12 hours and they would be very sore the next day. Others can do both legs in one day with time to spare.
Craig W and I are planning a strenuous trip in a couple of weeks, and another person with little experience is going with us. About a month ago, I encouraged this 3rd person to do a hike of 9,000' elevation gain/loss on the trail as a precursor. Then a couple of weeks ago a hike of 10 miles with a gain of 8,000. He did both well, and I am now comfortable taking him on the trip. I feel that if I take someone on a trip, that may not be able to do it, then that person becomes my responsibility. Craig I am not worried about, he will kick my butt :)